Retinal diseases

From Cat
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The image shows two areas of hemorrhage at different levels of the retina, and a focal bullous retinal detachment. All these lesions were immediately apparent with the use of indirect ophthalmoscopy, without the need for further magnification. The image provided through an indirect ophthalmoscopy lens is oriented upside-down and backwards; that is why the tapetum is located ventrally. This technique allowed the clinician to diagnose a bleeding disorder that could have led to blindness in a cat presented for an annual wellness examination.
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Retinal detachment. In this patient, the retina is so close to the lens that it can be seen without a fundic examination, indicating retinal detachment. Hypertensive retinopathy is the main differential with this signalment and examination finding. Rule-outs should include renal and cardiac disease, hyperthyroidism, and pheochromocytoma.
- Progressive retinal atrophy
  • Acquired retinal diseases
- Retinal hyperviscosity
- Diabetic retinopathy
- Lipaemia retinalis
- Retinal haemorrhage
- Retinal detachment
- Retinal hypertension

Retinal diseases are a relatively common ophthalmological disease of the cat.

The fundus of the eye is the posterior part of the globe viewed ophthalmoscopically and includes the appearance of the retina, superimposed over the underlying choroid abd sclera and divided into tapetal and non-tapetal portions, the optic disc (optic nerve head or papilla) and the retinal blood vessels. The feline fundus is more regular and shows less variation than any of the other domestic animals and certainly less than that of the dog.

The retina is usually considered to have ten layers. The outermost layer, the retinal pigment epithelium, which lies next to the choroid, is derived from the outer layer of the optic cup (ectoderm) whereas the remaining nine layers, the neurosensory retina, develop from the inner layer of the optic cup (also ectoderm). The pigment epithelium is pigmented except over the tapetum, as is the case in all animals with a tapetal fundus.

The vascular pattern of the feline retina is classified as holangiotic, as is the case in the dog, with a direct blood supply to most of the retina. There are three major pairs of cilioretinal arterioles and slightly larger, and usually less tortuous, veins which leave at or near the edge of the optic disc, and extend towards the periphery, leaving the central portion of the disc, unlike the dog, free from blood vessels. A central retinal artery is not usually present in this species but has been reported.

The classification of feline retinal diseases differs between authors and is complicated by the fact that many cases may have an unknown etiology, even after detailed investigation. Feline retinal disease is often associated with systemic disease and a full eye examination, in particular the fundus, should always be included as part of the clinical examination of the sick cat and in a significant number of cases may be of considerable aid in diagnosis[1].

The normal fundus

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The above images show normal feline fundi. The cat has a very well developed, highly reflective, cellular tapetum. The shape is triangular, the appearance granular and the colour usually yellow to green, sometimes blue. Incomplete tapetal development with many islets of colour on a pigmented background is rare in the cat, but has been recorded. Absence of the tapetum occurs in some blue-eyed, white, colour-dilute cats, in others the tapetum is thin with visible choroidal vessels similar to that in blue merle dogs. The area centralis, an area of maximum cone density similar to the fovea, is situated approximately 3mm lateral to the optic disc. This region is devoid of blood vessels and is sometimes a darker green colour. This part of the tapetal fundus is of particular importance in taurine deficiency retinopathy or feline central retinal degeneration. Peripapillary rings of pigment or hyper-reflectivity (conus) are often present or may be absent. The nontapetal fundus is usually heavily pigmented and dark grey-brown in colour. In breeds such as the Siamese and Himalayans, which lack ocular pigment, the nontapetal fundus may be described as tigroid with visible choroid vessels due to lack of pigment in the retinal pigment epithelium; choroid pigment may also be lacking in some blue-eyed, white individuals and a few cats have patches of subalbinism in the nontapetal area. The tapetal junction with the nontapetal area is usually clearly defined but occasionally tapetal islets may be present.

Retinal diseases are often diagnosed using an ophthalmoscope, where a light is passed onto the back of the eye, visualising the retina. Pictures below illustrate various diseases and their impact on normal retina appearance.

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Above figures show (left to right) advanced retinal disease, progressive retinal atrophy and retinal haemorrhage following trauma

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Above figures show (left to right) gross anaemia and two cases of taurine deficiency

References

  1. Barnett, KC & Crispin, SM Feline Ophthalmology (2002) WB Saunders, London